Hereditary complement (C) deficiency states in man form a major basis for this research. These genetic deficiencies are being evaluated in their own right (a) in order to elucidate the associations with infectious, rheumatic or immunologic diseases; (b) to determine linkages to many known genetic markers. In addition, various C-deficient sera are being utilized to extend knowledge on the biological role of C in host defense mechanisms and cellular injury in man, including studies of the nature of C-dependent chemotactic factors, serum opsonic activity for bacteria and yeasts, intracellular killing of bacteria, neutralization of enveloped viruses, C-dependent platelet aggregation and release reactions, and the ultrastructural basis of C injury to cell membranes. BIBLIOGRAPHIC REFERENCES: Rosenfeld, S.I., Kelly, M.E., and Leddy, J.P.: Hereditary deficiency of the fifth component of complement in man. I. Clinical, immunochemical, and family studies. J. Clin. Invest. 57 (June, 1976), in press. Rosenfeld, S.I., Baum, J., Steigbigel, R.T., and Leddy, J.P.: Hereditary deficiency of the fifth component of complement in man. II. Biological poperties of C5 deficient human serum. J. Clin. Invest. 57 (June, 1976), in press.